A variety of steel sheets are processed continuously by a continuous annealing system for continuously processing steel sheets by a heat treatment and an alloy hot dip galvanizing system for processing hot dip galvanized steel sheets by an alloying process. It is important to control the process such that the temperature of the steel sheet after a heat treatment process including heating and cooling is adjusted accurately to a desired temperature for stabilizing the mechanical characteristics, such as strength and elongation, and the plating characteristics, such as alloying degree, of different steel sheets.
Generally, a noncontact radiation pyrometer is used for measuring the temperature of the steel sheets continuously moving in those systems. The emissivity of the steel sheet, namely, an object of measurement, needs to be determined when the radiation pyrometer is used. Emissivity of the steel sheet is dependent on the physical properties, such as quality and surface characteristic, and temperature. Therefore, it is difficult to determine the emissivity of the steel sheet on the basis of those variable factors. Thus, a measurement error appears easily in the measurement of the temperature of the steel sheet and, consequently, control for accurately adjusting the temperature of the steel sheet to a desired temperature cannot be achieved.
Various measuring methods having the least possible susceptibility to the variation of the emissivity of the steel sheet have been proposed. Those measuring methods use multiple reflection on the basis of knowledge that multiple reflection increases an apparent emissivity.
A cavity method mentioned in Patent document 1 places two types of cylindrical cavities respectively having inside surfaces having high reflectivity nearly equal to that of a mirror surface near a steel sheet, compares radiant energy passed through a first cavity making multiple reflection and radiant energy passed through a second cavity without making multiple reflection to determine the temperature and thermal emissivity of the steel sheet. This cavity method needs the two cylindrical cavities, which needs a large space.
Measuring methods using multiple reflection between a steel sheet and a reflecting plate and not needing a large space have been proposed.
A measuring method mentioned in Patent document 2 places a reflecting plate at an inclination to a steel sheet, and takes a temperature indicated by a radiation thermometer as the temperature of the steel sheet, regarding radiant energy of multiple reflection between the steel sheet and the reflecting plate as blackbody radiant energy.
A measuring method mentioned in Patent document 3 determines the emissivity of a steel sheet on the basis of radiant energy determined by a measuring method similar to that mentioned in Patent document 2.
The measuring methods mentioned in Patent documents 2 and 3 (including the cavity method mentioned in Patent document 1) are based on the fact that the radiant energy of the reflecting plate is far less the radiant energy of multiple reflection when the reflecting plate is at an ordinary temperature or a low temperature and that the effect of the variation of the emissivity of the steel plate on the radiant energy of multiple reflection is ignorable when the reflecting plate has a high reflectivity, i.e., the emissivity is lower than that determined by the Kirchhoff's law, or when the reflectivity (emissivity) of the steel sheet is known. Therefore, the measuring methods mentioned in Patent documents 2 and 3 (including the cavity method mentioned in Patent document 1) need to maintain the reflectivity of the reflecting plate at a high level, i.e., to maintain the surface of the reflecting plate in a mirror surface, or to maintain the known reflectivity of the reflecting plate for a long time. Since the reflectivity (emissivity) of the reflecting plate changes due to the oxidation of the surface of the reflecting plate, it is difficult to maintain accuracy stably for a long time. Similar matters are mentioned in Patent document 5, which will be described later.
To solve those problems, a measuring method mentioned in Patent document 4 achieves the measurement of the temperature of a steel sheet regardless of the influence of the emissivity of the steel sheet by causing multiple reflection between the steel sheets. This measuring method uses a basic physical phenomenon that apparent emissivity increases when multiple reflection occurs within an object of measurement, does not use any reflecting plate and is not subject to the influence of the reflectivity (emissivity) of a reflecting plate. Since the apparent emissivity is approximately 1 even if the emissivity of the steel sheet varies, measurements obtained by this measuring method include a small error and this measuring method is scarcely subject to aging. However, this measuring method can be used for measurement only at a limited measuring position where parts of a steel sheet running between upper and lower hearth rollers of a vertical furnace face each other because this measuring method uses multiple reflection between parts of the steel sheet.
A measuring method mentioned in Patent document 5 was developed by incorporating improvements into the measuring method mentioned in Patent document 3. The measuring method mentioned in Patent document 5 intends to ensure the improvement of measurement accuracy by employing a function to maintain a reflecting plate at a fixed temperature even if the reflectivity (emissivity) of the reflecting plate changes with time.
As mentioned in Patent document 3, reflection needs to be repeated predetermined times between the reflection plate and the steel sheet for multiple reflection. The radiation pyrometer inevitably needs a large reflecting plate to ensure the necessary times of reflection when the radiation pyrometer is disposed at a small angle (the angle θ in FIG. 2 of Patent document 3). The measuring method mentioned in Patent document 5 needs to set the reflecting plate at a proper temperature according to the emissivity and temperature of the steel sheet and the emissivity of the reflection plate to control the steel sheet accurately at a desired temperature. Thus, it is difficult to control the steel sheet accurately and stably at a desired temperature for a long period of time.
Patent document 1: JP S54-85079 A
Patent document 2: JP 259-87329 A
Patent document 3: JP S59-111026 A
Patent document 4: JP S60-86432 A
Patent document 5: JP H5-203497 A